We have purified cGMP-inhibited cAMP phosphodiesterases (cGI PDEs) from rat and bovine adipose tissues and more recently from human platelets by chromatography on DEAE-Sephacel and an affinity column constructed by coupling the isothiocyanate derivative of the specific inhibitor cilostamide to aminoethyl agarose (CIT-agarose). An analogous cGI PDE is found in sarcoplasmic reticulum from human hearts; specific inhibitors of this PDE are being studied in clinical trials for treatment of certain kinds of cardiac failure. Following limited proteolysis (trypsin or endoproteinase LysC digests) of the purified platelet cGI PDE and separation of peptides by HPLC, partial sequence of a number of peptides was obtained. Antibodies have been raised against the purified PDE as well as against a synthetic peptide corresponding to a known sequence. Purified IgG fractions cross-react with cGI PDEs from human platelets and cardiac sarcoplasmic reticulum, and rat adipocytes. Based on amino acid sequences from several peptides, oligonucleotide probes have been constructed and are being utilized for screening of human erythroleukemic, cardiac, and fat cell cDNA libraries.